An aluminum electrolytic capacitor includes a capacitor element serving as a functional unit. The capacitor element is composed of an anode foil and a cathode foil, both made of aluminum foils, which are wound with a separator interpolated therebetween. Lead terminals are preliminarily secured to the anode foil and the cathode foil, respectively. A rubber sealing member is attached to the lead terminals, and after the separator has been impregnated with an electrolytic solution, the capacitor element is inserted into a cylindrical metal case having a bottom, and by tightly sealing the opening of the metal case with the sealing member attached thereto, an aluminum electrolytic capacitor is manufactured.
Since the characteristics of the aluminum electrolytic capacitor are closely related to an electrolytic solution to be used therein, it is necessary to properly select the electrolytic solution in order to improve the characteristics thereof. In particular, in the electrolytic solution, there have been demands for high conductivity and a long-term reliability for use under high temperatures, and an electrolytic solution prepared by dissolving an organic acid serving as a solute in a solvent mainly composed of γ-butyrolactone has been used.
However, γ-butyrolactone is a highly volatile material. Consequently, in the case using a sealing member made from natural rubber, ethylene-propylene-diene terpolymer or the like, the electrolytic solution permeates and drifts away as vapor. Therefore, it is difficult to properly maintain performance of the aluminum electrolytic capacitor in a high-temperature environment.
For this reason, there has been used a sealing member made from butyl rubber composed of a copolymer of isobutylene and isoprene, which reduces the amount of permeation and scattering of the electrolytic solution and exerts superior air-tightness. Moreover, with respect to the adhesion between the lead wire and the wall of the through hole and long life under high temperatures, it has been proposed to use a sealing member that is mainly composed of a three-component copolymer (partially cross-linked butyl rubber) of isobutylene, isoprene and divinylbenzene, and is obtained by subjecting this to peroxide cross-linking (Patent Literature 1).
Moreover, it has been proposed to form an inorganic thin film on the surface of a rubber sealing member, and to add oblate fine particles having a predetermined specific surface area as a filler for the rubber sealing member, so that heat resistance is improved so as to reduce evaporation/volatilization of the electrolytic solution (Patent Literatures 2 and 3).
Furthermore, in recent years, environmental problems have been attracting much attention, and upon assembling electronic parts, a lead-free soldering process has been applied. In order to allow an aluminum electrolytic capacitor to withstand such a working temperature of soldering, a structure has been proposed in which mica is added to the sealing member as a filler (Patent Literatures 4 and 5).
However, in a conventional aluminum electrolytic capacitor, in the case where a sealing member of partially cross-linked butyl rubber is used, or in the case where an elastic sealing member obtained by subjecting partially cross-linked butyl rubber to peroxide cross-linking is used, upon carrying out a life test at high temperatures, an electrolytic solution tends to be easily leaked near a through hole for a lead wire of the elastic sealing member.
Moreover, in the case where an inorganic thin film is formed on the surface of a rubber sealing member, or in the case where oblate fine particles having a predetermined specific surface area are added thereto as a filler for the rubber sealing member, evaporation/volatilization of an electrolytic solution can be reduced. However, in contrast to a prolonged capacitor life of 2000 hours to 10000 hours at 105° C., the rubber sealing member allows the electrolytic solution to permeate therethrough, failing to reduce evaporation/volatilization of the electrolytic solution.